1. Field of the Invention
The present invention relates to a fuel cell power generating apparatus, the operating method of a fuel cell power generating apparatus, and the combined battery of a fuel cell power generating apparatus.
2. Description of the Related Art
A high performance secondary battery and a fuel cell attract much attention as a power source of a portable electronic appliance supporting an information society and as a key element of an electric automobile and a power storing system for coping with air pollution and “global warming”. Particularly, a PEM (PEFC, solid electrolyte fuel cell) using hydrogen and oxygen as a fuel is considered to be a hopeful application of a secondary battery and fuel cell for electric automobiles. To be more specific, the fuel cell is considered to be adapted for an electric automobile because a high output can be obtained by using clean energies of hydrogen and oxygen, and because the output can be restored by replenishing the fuel. However, a fuel cell is defective in that the output is lowered relative to the large change in the load current, making it difficult to generate the power required for sudden acceleration, in an electric automobile. On the other hand, an electric automobile utilizing a lithium ion secondary battery alone for the running has been put to practical use. However, it is considered difficult to use a lithium ion secondary battery alone as the power source of an electric automobile because, further improvements in terms of safety and output are required, in that the output cannot be restored sufficiently even if the electrolyte is replenished. Under the circumstances, a hybrid battery (combined battery) utilizing the features of both a lithium ion secondary battery and a fuel cell has become important for providing a satisfactory electric automobile. Further, in order to achieve a reduction in volume of the fuel used in PEM, a method using a compressed hydrogen (250 atm), a liquid hydrogen, or a hydrogen storage alloy is being studied. Under the circumstances, a direct methanol fuel cell (DMFC), in which protons are directly extracted from methanol for performing power generation, attracts attention, although the output of a DMFC is lower than that of a PEM. Further, since a DMFC permits reducing the volume of the fuel, it is considered possible to apply a DMFC (direct methanol fuel cell) to a portable electronic appliance, leading to high expectations in the applications of DMFCs in various fields.
FIG. 1 schematically shows the construction of a typical direct methanol fuel cell. The electromotive section of the direct methanol fuel cell comprises an anode electrode including an anode current collector 1 and an anode catalyst layer 2, a cathode electrode including a cathode current collector 3 and a cathode catalyst layer 4, and an electrolyte membrane 5 interposed between the anode electrode and the cathode electrode. An anode fluid channel plate 6 is arranged on the side of the anode current collector 1. As shown in FIG. 2, an anode fluid channel 9 having a methanol supply port 7 and a methanol discharge port 8 is formed on the anode fluid channel plate 6. A methanol aqueous solution container 10 housing a methanol aqueous solution is connected to the methanol supply port 7 with a pump 11 interposed therebetween. On the other hand, a cathode fluid channel plate 12 is arranged on the side of the cathode current collector 3. A cathode fluid channel 15 including an oxidizing agent supply port 13 and an oxidizing agent discharge port 14 is formed in the cathode fluid channel plate 12. An oxidizing agent supply mechanism 16 which supplies an oxidizing agent such as air is connected to the oxidizing agent supply port 13.
The electrolyte membrane 5 is formed of, for example, a Nafion® membrane having a high proton conductivity. On the other hand, the catalyst used in the anode catalyst layer 2 is formed of, for example, PtRu low in poisoning, and the catalyst used in the cathode catalyst layer is formed of, for example, Pt.
In the direct methanol fuel cell of the construction described above, a methanol aqueous solution is supplied to the anode catalyst layer 2 so as to bring about a catalytic reaction for generating protons. The protons thus generated are migrated though the electrolyte membrane 5 so as to carry out a reaction with oxygen supplied to the cathode catalyst layer 4 in the presence of the catalyst so as to perform the required power generation.
In order to improve the output of the direct methanol fuel cell, it is necessary to maintain a high electromotive force to a high load current. In order to obtain a high load current, it is necessary to increase the amount of methanol supplied to the anode catalyst layer per unit time. However, since the methanol aqueous solution passes through the Nafion® membrane, the methanol aqueous solution that was not used in the reaction carried out within the anode catalyst layer is migrated to reach the cathode catalyst layer. As a result, the reaction equal to that carried out in the anode catalyst layer is carried out in the cathode catalyst layer, which generates a back electromotive force, which is a cross-over overvoltage that brings about a problem that the electromotive force of the direct methanol fuel cell is decreased. This cross-over overvoltage is rendered serious with an increase in the methanol concentration. If a methanol aqueous solution having a concentration exceeding 5M is supplied to the anode catalyst layer, the output of the direct methanol fuel cell is markedly decreased. It is also desirable to set the concentration of the methanol aqueous solution at a level not higher than 5M in order to suppress the deterioration of the electrolyte membrane such as a Nafion® membrane. Such being the situation, it is desirable to set the concentration of the aqueous solution of methanol used as a fuel at a level not higher than 5M in order to operate the direct methanol fuel cell.
As a method of lowering the cross-over overvoltage, it is conceivable to have all the methanol supplied to the anode catalyst layer consumed in the anode catalyst layer so as to prevent methanol from being migrated into the cathode catalyst layer. For allowing all the methanol consumed in the anode catalyst layer, it is conceivable to increase the catalytic activity in the anode catalyst layer or to increase the amount of the catalyst supported by the carrier. However, it is impossible to increase the catalytic activity or to increase amount the supported catalyst in the anode catalyst layer used nowadays. Also, an electrolyte membrane inhibiting the migration of the methanol solution that is not used in the anode catalyst layer is also being developed. However, the proton conductivity of the electrolyte membrane is rendered poor, which lowers the output in many cases. Such being the situation, it is desirable to lower the concentration of the methanol aqueous solution used as a fuel in order to lower the cross-over overvoltage. However, if a fuel of a low concentration is used, it is necessary to enlarge the fuel container, resulting in failure to utilize sufficiently the feature of the direct methanol fuel cell.
As described above with reference to FIG. 1, the ordinary direct methanol fuel cell is constructed such that a methanol aqueous solution is supplied by the pump 11 to the anode fluid channel plate 6. Also, as shown in FIG. 2, the methanol aqueous solution supplied by the pump 11 flows along the groove portion (anode fluid channel 9) of the fluid channel plate 6 via the methanol supply port (inlet) 7 of the anode fluid channel plate 6. The convex portion of the fluid channel plate 6 is in contact with the anode current collector 1 formed of an anode carbon paper such that the methanol aqueous solution flowing through the anode fluid channel 9 permeates into the anode current collector 1, which supplies the methanol aqueous solution into the anode catalyst layer 2.
However, all the methanol aqueous solution flowing along the anode fluid channel plate 6 does not necessarily permeate into the anode current collector 1. In practice, the methanol aqueous solution is partly discharged to the outside through the methanol discharge port (outlet) 8 of the fluid channel plate 6. It follows that the utilization efficiency of the methanol aqueous solution in the container 10 is generally low. For improving the utilization efficiency, it is attempted to improve the construction of the fluid channel plate. However, a high improvement in the utilization efficiency has not yet been achieved. It is also conceivable to assemble a system that the methanol aqueous solution discharged through the methanol discharge port (outlet) 8 of the anode fluid channel plate 6 is returned to the container 10. However, methanol and water are consumed at a ratio of 1:1 within the anode catalyst layer 2. It follows that, if the methanol aqueous solution discharged through the anode fluid channel plate 6 is brought back to the container 10, the concentration of the methanol aqueous solution within the container 10 is gradually lowered. As a result, a methanol shortage is brought about inside the battery, giving rise to the problem that the electromotive force is rapidly decreased.
As described above, it is desirable to use a methanol aqueous solution having a concentration not higher than 5M. However, if a dilute methanol aqueous solution is used as a fuel, it is necessary to increase the volume of the container. In addition, the methanol shortage tends to take place for carrying out the reaction inside the cell. It follows that it is necessary to supply promptly the methanol aqueous solution from within the methanol aqueous solution container. If such an operation is carried out, it is certainly possible to increase the output of the fuel cell because the cross-over overvoltage can be lowered. However, the pump output for supplying methanol is also increased, giving rise to the problem that the output of the power generating apparatus including the fuel cell and the pump is rendered low.
As described above, contradictory situations are brought about. Specifically, it is desirable in view of the fuel supply to supply a thick methanol aqueous solution at a low flowing speed. In view of the output of the power generating apparatus, however, it is desirable to supply a thin methanol aqueous solution at a high flowing speed. Such being the situation, in order to reduce the volume of the fuel container and to obtain a high output, it is necessary to supply a methanol aqueous solution of an optimum concentration at an optimum flowing speed. The optimum concentration and the optimum flowing speed of the methanol aqueous solution are also dependent on the construction of the electromotive force of the fuel cell and, thus, it is very difficult to research the optimum concentration and the optimum flowing speed of the methanol aqueous solution. As a matter of fact, the optimum concentration and the optimum flowing speed of the methanol aqueous solution are not yet understood.